This invention relates to the process for pretreating hydrocarbon feed stocks that is described in U.S. Pat. No. 4,243,514 to David B. Bartholic, entitled "Preparation of FCC Charge from Residual Fractions." The entire disclosure of that patent is incorporated herein by cross-reference thereto. This invention particularly relates to a novel catalytically inert (or substantially inert) fluidizable solid that is derived from equilibrium fluid cracking catalyst particles and to the use of such material as a contact agent in the process for pretreating hydrocarbon feed stocks that is described in the aforementioned Bartholic patent.
In U.S. Pat. No. 4,243,514, a process is described for increasing the portion of heavy petroleum crudes which can be utilized as the hydrocarbon feed stocks for fluid catalytic cracking ("FCC") processes to produce premium petroleum products, particularly motor gasoline of high octane number or high quality heavy fuel. The heavy ends of many crudes are high in Conradson Carbon residues (sometimes reported as Ramsbottom Carbon residues) and metal values, such as nickel and vanadium, as well as salts, such as sodium salts, which are undesirable in FCC feed stocks and in products such as heavy fuel. The process of U.S. Pat. No. 4,243,514 provides an economically attractive method for selectively removing and utilizing these undesirable components from whole crudes, as well as from the bottom fractions or residues of atmospheric and vacuum distillations of whole crudes, commonly called atmospheric and vacuum residua or "resids". In this regard, terms such as "residual stocks" and "resids" are used in a somewhat broader sense than is usual to include any petroleum fraction remaining after fractional distillation of petroleum to remove some of its more volatile components. In that sense, "topped crude", remaining after distilling off gasoline and lighter fractions, is a resid. The undesirable high Conradson Carbon (low hydrogen content) compounds, such as polynuclear aromatic compounds, and metal-containing compounds, as well as salts, present in crudes (e.g., whole crudes or resids) tend to be concentrated in the resids because most of them have low volatility.
When first introduced to the petroleum industry in the 1930's, the FCC process constituted a major advance over previous processes for increasing the yield of motor gasoline from petroleum to meet ever increasing demands. The FCC process was adapted to produce abundant yields of high octane naphtha from petroleum fractions boiling above the gasoline range, upwards of about 400.degree. F. Greatly improved FCC process have since been developed by intensive research efforts, and plant capacity has expanded rapidly up to the present, so that the catalytic cracker is today the dominant unit or "workhorse" of a petroleum refinery.
As installed capacity of FCC processes has increased, there has been increasing pressure to charge, as feed stocks to FCC units, greater proportions of crudes. However, two major factors have opposed that pressure, namely, the Conradson Carbon residues and metal values in the crudes. As the Conradson Carbon residues and metal values have increased in crudes charged to FCC processes, capacity and efficiency of catalytic crackers have been adversely affected. Also, the quality of heavy fuels, such as Bunker Oil and heavy gas oil, produced by FCC processes has also been adversely affected as it has become necessary to make these fuels from crudes of high Conradson Carbon residues and high metal values.
The effect of high Conradson Carbon residues in hydrocarbon feed stocks for FCC processes has been to increase the portion of the feed stocks converted to "coke" deposits on the FCC catalysts. As coke has built up on the FCC catalyst, the active surfaces of the catalysts have been masked and rendered inactive for the desired catalytic cracking. It has been conventional practice to burn off the inactivating coke with air to "regenerate" the active surfaces, after which the catalysts have been returned in cyclic fashion to the reaction stage for contact with, and cracking of, additional feed stocks. The heat generated in the regeneration stage has been recovered and used, at least in part, to supply the heat of vaporization of the feed stocks and the endothermic heat of the cracking reaction. The regeneration stage has operated under a maximum temperature limitation to avoid heat damage to the catalysts. As the Conradson Carbon residues in feed stocks have increased, coke burning capacity has become a bottle-neck which has forced a reduction in the rate of charging the feed stocks to FCC units. In additionm, part of the feed stocks has inevitably had to be diverted to undesirable reaction products.
Metal values, such as nickel and vanadium, in hydrocarbon feed stocks for FCC processes have tended to catalyze the production of coke and hydrogen in FCC units. Such metals also have tended to be deposited on FCC catalysts, as the molecules in which they occur in the feed stocks are cracked, and to build up on the catalysts. This has further increased coke production with its accompanying problems. Excessive hydrogen production also has caused a bottle-neck problem in processing lighter ends of cracked products through fractionation equipment to separate valuable components, primarily propane, butane and the olefins of like carbon number. Hydrogen, being incondensible in the "gas plant", has occupied space as a gas in the compression and fractionation train and has tended to overload the system when excessive amounts are produced by high metal content catalysts. Conventional practice is to withdraw equilibrium fluid cracking catalyst periodically from circulating catalyst inventory to maintain catalytic activity and selectivity at desired levels. Fresh catalyst is added to compensate for both withdrawn equilibrium catalyst and catalyst fines resulting from attrition of catalyst particles during use. Feed stocks high in metals generally necessitate high rates of withdrawal of equilibrium catalyst and/or reducton in feed stock charge rates to maintain FCC units and their auxiliaries operative.
These problems have long been recognized in the art, and many ways, discussed in U.S. Pat. No. 4,243,514, have been proposed to remove the high Conradson Carbon and metal-containing components from hydrocarbon feed stocks, such as resids, before they are used in FCC processes.
By the pretreatment process in U.S. Pat. No. 4,243,514, high Conradson Carbon and metal-containing components, as well as salts, can be economically removed from a hydrocarbon feed stock, containing the highest boiling components of a crude, before charging the feed stock to an FCC unit or a hydroprocessing unit. In this pretreatment process, the feed stock is subjected to a selective vaporization step in which there is a high temperature, short hydrocarbon residence time contact in a confined rising vertical column between the feed stock and a hot fluidized solid contact material. The contact material serves as a heat transfer medium and acceptor of unvaporized material from the feed stock. The contact material is essentially inert in the sense that it has low catalytic activity for inducing cracking of the feed stock. There is an expressed preference for using contact material that has a much lower surface area relative to its weight than conventional FCC catalysts.
During the selective vaporization step, most of the feed stock is vaporized by the high temperature contact with the contact material. However, the majority of the high Conradson Carbon and metal-containing components of the feed stock, as well as salts in the feed stock, are not vaporized by the high temperature contact with the contact material but are instead deposited on the surface of the contact material. The contact material, on which the unvaporized portions of the feed stock have been deposited, is then subjected to a combustion step in which the combustible portions of the deposits on the contact material are oxidized to generate heat which is imparted to the contact material. The so-heated contact material is then recycled and contacted with additional feed stock. By this process, the heat required for the selective vaporization step is generated by oxidation of the combustible deposits on the contact material, including the combustible high Conradson Carbon and metal-containing components of the feed stock.
The Bartholic patent teaches that fluidizable solid contacting agent suitable for the selective vaporization step is essentially inert in the sense that it induces minimal cracking of heavy hydrocarbons by a standard microactivity test conducted by measurement of amount of gas oil converted to gas, gasoline and coke by contact with the solid in a fixed fluidized bed. Charge in that test is 0.8 grams of mid-Continent gas oil of 27.degree. API contacted with 4 grams of catalyst during 48 second oil delivery time at 910.degree. F. This results in a catalyst to oil ratio of 5 at weight hourly space velocity (WHSV) of 15. By that test, the solid employed in the process of U.S. Pat. No. 4,243,514 exhibits a microactivity less than 20, preferably about 10. The preferred fluidizable solids, according to the teaching of the patent, are microspheres of calcined kaolin clay. Other solids disclosed in the patent include low surface area forms of silica gel and bauxite. A variety of other solids of low catalytic activity are mentioned at col. 5. General criteria for selection include low cost, low catalytic activity, availability in the form of inert fluidizable particles and low surface area. The patent takes note of the fact that the desired low surface area is considerably below that of commercial fluid cracking catalysts.
As described in U.S. Pat. No. 4,243,514, decarbonized, demetallized resid is good quality hydrotreating, hydrocracking or FCC charge stock and may be transferred to the feed line of an FCC reactor operated in the conventional manner. Spent catalyst from the FCC reactor passes by a standpipe to a conventional FCC regenerator while cracked products leave reactor by transfer line to fractionation for recovery of gasoline and other conversion products. Hot regenerated FCC catalyst is transferred from an FCC regenerator by a standpipe for addition to the FCC reactor.
The economics of the selective vaporization process of U.S. Pat. No. 4,243,514 is dependent upon the cost, availability and performance characteristics of the inert fluidizable solid. When the selective vaporization step is carried out at a refinery site that includes one or more FCC units, equilibrium fluid cracking catalyst particles are made available when the material is withdrawn from the cracking units in order to maintain the activity and selectivity of the circulating cracking catalyst inventory at acceptable levels. Virtually all present refineries utilize zeolitic cracking catalysts. Properties and characterization of commercial zeolitic cracking catalysts appear in a monograph "Fluid Catalytic Cracking Catalysts," Paul B. Venuto and E. Thomas Habib, Jr., Vol. I, published by Marcel Dekker, Inc. pages 30-43 (1979).
Typical equilibrium zeolitic FCC catalysts are not suitable for use in the selective vaporization process of U.S. Pat. No. 4,243,514 because of their high residual level of cracking activity and high surface area. A comparison of representative fresh and equilibrium fluid zeolite FCC catalyst is reported in the monograph above cited at page 46. The equilibrium catalyst contained fairly low levels of metals (i.e., 259 ppm of V+Ni+Cu). Catalytic activity ("Microactivity") was 85% for the fresh zeolitic catalyst and 73% for equilibrium zeolitic catalyst; carbon and hydrogen factors were 0.6 and 0.2, respectively, for fresh catalyst and 0.6 and 0.7, respectively, for equilibrium catalyst. Surface area decreased from 335 to 97 m.sup.2 /g. when the fresh catalyst reached equilibrium state. Pore volume decreased from 0.60 to 0.45 cm.sup.3 /g.
While the equilibrium catalyst was less active and had lower surface area than did the fresh catalyst, the former material does not meet the performance criteria for a contact material for use in the process of the Bartholic patent. However, equilibrium catalyst does have desirable density and attrition-resistance and it finds use as the active contact material for starting-up FCC units which cannot tolerate the activity of fresh catalyst. However, in some refineries there is an excess of available equilibrium catalyst. Such excess may be supplied to other refineries for start-up. A notable exception is equilibrium catalyst in which the metals level is high, e.g., 1000 ppm V+Ni+Cu. These heavily contaminated catalysts are generally not useful for start-up. In effect such equilibrium catalyst is a waste material, finding utility as landfill or other low-value disposition.
Various suggestions have been made to divert either equilibrium cracking catalyst withdrawn from a catalyst regenerator or catalyst fines to other points in a refinery for the purpose of pretreating cracker feedstock in one way or another. Some pretreatments involve liquid-solid contact in a first stage carried out either under pressure or relatively low temperature to maintain feed stock in liquid state. For example, nitrogen bases, sulfur or salts are removed before feed stock is catalytically cracked. Other pretreatments, generally involving vapor-solid contact, utilize the minimized residual cracking activity of used catalyst in a first stage mild cracking operation. Reference is made to the following patents:
U.S. Pat. No. 2,944,002--Faulk PA1 U.S. Pat. No. 2,689,825--McKinley PA1 U.S. Pat. No. 2,614,068--Healy et al. PA1 U.S. Pat. No. 2,605,214--Galstaum PA1 U.S. Pat. No. 2,521,757--Smith PA1 U.S. Pat. No. 2,541,267--Mills, Jr. et al. PA1 U.S. Pat. No. 2,461,958--Bonnell PA1 U.S. Pat. No. 2,378,531--Becker
In the Smith patent, the activity of spent catalyst from a second stage cracking may be controlled if necessary by steaming or calcination before utilization in first stage cracking. However, the intent of patentee is to utilize the ability of spent catalyst to crack feed stock.
While equilibrium FCC catalyst from present day refineries would seem to provide a low cost source of fluidizable attrition resistant particles potentially useful in pretreating feedstocks by selective vaporization, the residual activity and, in most cases, high surface area, rule out this alternative. It is know that sodium compounds such as sodium chloride are poisons for FCC catalysts. Note the Becker patent, supra. Chloride salts, however, tend to increase coke make. Therefore, deactivation of equilibrium catalyst by addition of sodium chloride will result in a material that would be of limited use as the contact material in the pretreatment process of the Bartholic patent. Conversion of feed stock to coke would reduce the portion of feed stock constituting valuable FCC feedstock. Sodium hydroxide in FCC feedstock is also known to deactivate zeolitic cracking catalyst. We have found that addition of caustic to equilibrium catalyst particles followed by thermal treatment to sinter the particles may result in significant decrease in catalytic activity. However, coke make is high as compared to coke make using fluidizable particles of calcined kaolin clay unless high levels of caustic are used or extremely high calcination temperature is employed.